This invention relates to a common control switching system including a plurality of functional block circuits and, more particularly, to a control circuit for selecting desired ones of the functional block circuits and putting the selected functional block circuits into operation.
It is already known that a common control switching system including wired logic generally comprises a plurality of functional block circuits in the common control equipment. Examples of the functional block circuits are call detection circuits, line selection circuits, trunk equipment selection circuits, trunk class specification circuits, channel selection circuits, speech channel switch driving circuits, and busy test circuits. In this connection, it is to be remembered that a common control switching system comprises a common control equipment and a plurality of connection operation equipment, which are the peripheral equipment in some sense. Examples of the connection operation equipment as called herein are line circuits, links, switches, trunk equipment, and registers. The connection operation equipment may thus be classified into a plurality of groups according to the connection operations carried out thereby. The connection operation equipment forming each of the groups are coupled to at least one of the functional block circuits. Each connection operation equipment of the line circuit group is capable of delivering a signal to the common control equipment in response to a call originating with a local subscriber. Each connection operation equipment of the register group is capable of delivering a signal of one of predetermined levels or codes to the common control equipment in compliance with a numerical signal, such as a series of dial pulses or a multifrequency signal, registered therein. The predetermined levels are representative of types of connection known or pretranslated from the numerical signals. It may therefore be said that each connection operation equipment of at least two of the groups comprises an initiation signal circuit, responsive to a call arriving thereat and requesting a connection to be established by the switching system, for delivering an initiation signal to the common control equipment.
It has been the practice that the functional block circuits are accompanied by fixed wired logic which serve altogether as a control for the functional block circuits and, responsive to an initiation signal, select some of the functional block circuits required to establish a connection requested by a call responsive to which the initiation signal has been produced. Under the control of the wired logic, the selected functional block circuits in turn select connection operation equipment one from each group of connection operation equipment coupled to the selected functional block circuits, and make the selected connection operation equipment sequentially carry out their respective connection operations to establish the requested connection. It should therefore be noted that the connection of a type, as called herein, such as a dial tone connection, a tone trunk connection, a recorded announce trunk connection, a charging trunk connection, a dial pulse incoming trunk connection, an outgoing connection, an incoming connection, an intra-office connection, or a transit connection, is established by a sequence of connection operations caused to be carried out by a set of selected functional block circuits.
Because of the fixed wired logic, conventional common control switching systems of the type described are capable of accomplishing only the connections of the types for which the wired logics are designed. Changes and/or additions of the wired logics affect the mutual effects between some of the functional block circuits. It has therefore been troublesome to change the connection control capablilities of the wired logics and/or to add some new connection control capabilities and/or services.
On the other hand, electronic switching systems of the stored program types are known. Although very flexible, such electronic switching systems have been expensive and particularly uneconomical for medium and small-scale central offices.